Growth and Characterization of Ultrathin Vanadium Oxide Films on HOPG

Abstract

Integration of graphene into various electronic devices requires an ultrathin oxide layer on top of graphene. However, direct thin film growth of oxide on graphene is not evident because of the low surface energy of graphene promoting three-dimensional island growth. In this study, we demonstrate the growth of ultrathin vanadium oxide films on a highly oriented pyrolytic graphite (HOPG) surface, which mimics the graphene surface, using (oxygen-assisted) molecular beam epitaxy, followed by a post-annealing. The structural properties, surface morphology, and chemical composition of the films have been systematically investigated by in situ reflection high-energy electron diffraction during the growth and by ex situ techniques, such as atomic force microscopy, scanning tunneling microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). Crystalline monolayer vanadium oxide can be achieved on HOPG by systematically tuning the deposition time of V atoms and by subsequent annealing at 450 °C in controlled atmospheres. Increasing the partial pressure of O₂ during the deposition seems to decrease the mobility of V atoms on the graphitic surface of HOPG and promote the formation of a two-dimensional (2D) vanadium oxide. The obtained oxide layers are found to be polycrystalline with an average grain size of 15 nm and to have a mixed-valence state with mainly V⁵⁺ and V⁴⁺. Moreover, XPS valence band measurements indicate that the vanadium oxide is insulating. These results demonstrate that a 2D insulating vanadium oxide can be grown directly on HOPG and suggest vanadium oxide as a promising candidate for graphene/oxide heterostructures.

Keywords: 2D; HOPG; X-ray photoelectron spectroscopy; atomic force microscopy; molecular beam epitaxy; scanning tunneling microscopy; thin films; vanadium oxide.

Figure 1

RHEED pattern of (a) clean HOPG (after degassing); (b) sample c₁, 0.45 nm V deposition on HOPG without O₂; (c) sample d₁, 0.45 nm V deposition on HOPG with O₂; (d) annealed sample c₂; and (e) annealed sample d₂. The yellow, orange and red solid lines indicate the RHEED streak spacings of V, VO_x and HOPG, respectively. The green dotted lines indicate that the RHEED spacings of HOPG are the same before and after V deposition. The blue arrows mark the weak streak patterns from VO_x.

Figure 2

AFM images with size 1 × 1 µm²: (a–d) as-grown samples a₁–d₁, and (e–h) corresponding annealed samples a₂–d₂. The RMS roughness of the samples is provided in Table S2. Insets show AFM scans at higher magnification (100 × 100 nm²).

Figure 3

STM characterization of the VO_x films after post-annealing. (a) Large-scale and (b) small-scale topographies of the annealed sample b₂. Dotted lines indicate the orientation of various domains. (c) Large-scale topography and (d) atomic-resolution STM image of the annealed sample c₂. (e) Large-scale topography image of the annealed sample d₂ and (f) the derived height histogram. (g) Atomic-resolution STM image of the ‘high’ islands in (e). Atomic-resolution images (d,g) are optimized by applying Fourier-transform-filtering. Scanning bias voltage and current are (a,b) 0.7 V, 0.6 nA; (c) 0.8 V, 0.6 nA; (d) 0.8 V, 1.0 nA; (e) 0.9 V, 0.6 nA; (g) 0.7 V, 0.7 nA, respectively.

Figure 4

XPS core-level (left) O1s-V2p and (middle) C1s spectra, and (right) valence band spectra of sample b₁ (as-grown) and b₂ (annealed).